Date Formatting
Date Formats are set by locale on an individual level:
Select your main account Locale in Setup --> System Settings
Select Individual Date formats
Staff
Setup --> Staff
Select Edit Locales to bulk edit on the listing page.
Edit a Staff Member (pencil icon)
Select the Date Format in the top section.

Owners
Setup --> Owners
Edit an Owner (pencil icon)
Select the Date Format in the top section.

Vendors
Setup --> Vendors
Edit a Vendor (pencil icon)
Select the Date Format in the top section.

The Date Format setting in Setup --> System Settings will prefill the Staff, Owners and Vendors when adding new.
Note: The Date Format and the Dashboard Language are two separate settings.